Most cheap USB Type-C cables can't carry the full 3.0A required by the spec.

One particularly conscientious Google engineer, Benson Leung, is currently on an unusual mission: he's slowly working his way through a bunch of USB Type-C cables and adaptors stocked by Amazon, to check whether they are actually up-to-spec and capable of charging his Chromebook Pixel.

First things first: of the ten USB Type-C products that Leung has reviewed, only three of them were fully specs-compliant and capable of charging his Pixel. The three good cables (Belkin, iOrange-E, Frieq) were invariably more expensive (about £15/$20) than the seven duff ones (£6/$10). Obviously there may be some cheap cables that do fulfil the full USB Type-C specification, but Leung hasn't found one yet.

Enlarge/ One of the offending micro-USB-to-Type-C adaptors that lacks the necessary hardware to comply with the Type C 1.1 spec.

The USB Type-C 1.1 specification allows for power delivery of up to 3A, which is enough juice to charge a laptop like the Chromebook Pixel. Previous USB specs, though, only allowed for power delivery of between 900mA and 1.5A. According to Leung, the problem is mostly related to how the cables deal with going from older Type-A or Micro/Mini connectors to the new Type-C connector.

I bought these two USB-C to Micro USB adapters from TechMatte and found they do not work properly with the Chromebook Pixel.

Upon closer inspection by our engineering team here, we have determined that this adapter is not correctly following the USB Type C specification...

Specifically, these adapters do not charge the Chromebook Pixel 2015 because the adapters leave the C-C lines floating, where the specification requires a Rp pullup to Vbus to identify the cable as a legacy adapter or cable... In other words, since you are creating a USB Type-C plug to a USB 2.0 Micro-B receptacle assembly, you must use a resistor of value 56kΩ as a pullup to Vbus. This cable does not do this.

In short, the cable doesn't fulfil the full Type-C 1.1 spec. Or, in Leung's own words, "If you are a consumer looking for a cable that is compatible with Pixel, do not use this one."

Further Reading

Most of Leung's reviews follow a similar pattern. In his review of the CableCreation Micro-to-Type-C adaptor, he calls them out for using a 10kΩ resistor instead of a 56kΩ. "By using this cable, your phone, tablet, or laptop computer may attempt to draw 3A, which may be more than the micro-b to A cable you attach to this adapter may be able to handle. This may cause damage to whatever cable, hub, pc, or charger you plug into this," Leung concludes.

If you take a broader look at the reviews on Amazon, it seems most of these cheaper cables still work when charging a smartphone—but using them to charge a higher-power device such as the Chromebook Pixel (and perhaps the new MacBook) probably won't work, and may be quite dangerous.

For the time being, until cheap third-party USB Type-C cables and adaptors become a little more mature, it's probably a good idea to stick to official products from Apple, Google, or products created by reputable third parties. If you're a Chromebook Pixel user, Leung has helpfully provided instructions on how you can check whether a Type-C cable/adaptor is up-to-spec.

For more information on the (somewhat confusing) Type-C connector, and how it dovetails into other standards such as USB 3.0 and USB Power Delivery, read our full explainer.

Sebastian Anthony
Sebastian is the editor of Ars Technica UK. He usually writes about low-level hardware, software, and transport, but it is emerging science and the future of technology that really get him excited. Emailsebastian@arstechnica.co.uk//Twitter@mrseb